1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display (LCD), which comprises an active panel including thin film transistors (TFTs) and pixel electrodes, i.e., to an active matrix liquid crystal display (AMLCD), and to the structure of a liquid crystal display made by the same method. Particularly, the present invention relates to a method of enhancing the electrical contact between the conductive materials for an enhanced screen quality and the structure of an LCD made by the same method.
2. Description of the Related Art
Among display devices for showing visual images on a screen, cathode ray tube (CRT) display devices, which have been used in general, are nowadays being replaced by thin film type flat panel displays which are thin, light and easily usable at any place. In particular, active research activities have been focusing on the development of liquid crystal displays because of their high resolution and fast response time suitable for display of motion picture images.
A liquid crystal display works by using polarization and optical anisotropy of a liquid crystal. The orientation of liquid crystal molecules is controlled by applying an electromagnetic field to the molecules which are arrayed in one direction and have polarization because of their long and thin shape. When controlling the orientation of liquid crystal molecules, transmission of a light through the liquid crystal is achieved due to the anisotropy of the liquid crystal. This principle is applied to a display device. Because active matrix liquid crystal displays (AMLCDs), which have TFTs arranged in a matrix pattern and pixel electrodes connected to the TFTs, provide high quality images and natural colors, they are actively studied. The structure of a conventional liquid crystal display will now be described.
The conventional liquid crystal display comprises two panels, on which various elements are placed, and liquid crystal between the two panels. One panel of the LCD includes elements reproducing colors, which is called a color filter panel. The color filter panel has color filters of red (R), green (G) and blue (B) which are sequentially arranged and correspond to pixels formed in a matrix pattern on a transparent substrate. Among these color filters, very thin black matrixes are formed in a lattice pattern. They prevent the mixture of colors at boundaries of the color filters. A common electrode covers the color filters, which functions as one electrode generating an electric field applied to the liquid crystal.
The other panel includes switching elements and bus lines which generate the electric field for driving the liquid crystal. It is called an active panel. The active panel has pixel electrodes which are formed on the transparent substrate. The pixel electrodes are opposite to the common electrode formed on the color filter panel, and functions as the other electrode generating the electric field applied to the liquid crystal. Signal bus lines run along the column direction of the array of the pixel electrodes, and data bus lines run along the row direction of the array of the pixel electrodes. At a corner of the pixel electrode, a TFT, which applies electromagnetic field to the pixel electrode, is formed. A gate electrode of the TFT is connected to the signal bus line (gate bus line), and a source electrode is connected to the data bus line (source bus line). A drain electrode of the TFT is connected to the pixel electrode. A gate pad and a source pad, which function as terminals receiving external signals, are formed at the end portions of the gate bus line and the source bus line, respectively.
When an external electric signal, which is applied to the gate pad, is sent to the gate electrode through the gate bus line, an electrical picture data, which is applied to the source pad, is sent to the source electrode through the source bus line and to the drain electrode. In the case that the electric signal is not applied to the gate electrode, the electrical picture data, which is applied to the source electrode over the gate electrode, is not sent to the drain electrode. Accordingly, whether the data signal is applied to the drain electrode is determined by controlling the signal to the gate electrode. Therefore, applying the data signal to the pixel electrode, which is connected to the drain electrode, is artificially controlled. In other words, the TFT functions as a switch for driving the pixel electrode.
These two panels are joined with a certain distance (called "cell gap"), and the liquid crystal is injected there-between. Finally, polarizing plates are attached to the outer surfaces of the two panels, and the liquid crystal panel of the LCD is completed.
There are various manufacturing methods and structures of the liquid crystal displays. Active research has been carried out in order to improve the efficiency of the LCD and to reduce the manufacturing costs. Conventional manufacturing method and structure of the LCD, which are related to the present invention, will now be described with reference to FIG. 1, which is a plan view showing the conventional LCD, and FIG. 2, which is a cross-sectional view showing a conventional manufacturing process taken along line II--II of FIG. 1.
A metal including aluminum or aluminum alloy is deposited on a transparent glass substrate 1. A gate electrode 11, a gate bus line 13 and a gate pad 15 are formed by patterning the metal. The gate electrode 11 is disposed at the corner of each pixel. The gate line 13 connects the gate electrodes 11 arrayed in the row direction. The gate pad 15 is formed at the end of the gate line 13 (FIG. 2a).
An inorganic insulating material such as silicon oxide(SiO.sub.x) or silicon nitride (SiN.sub.x) is deposited on the gate electrode 11, the gate line 13, and the gate pad 15 to form a gate insulating layer 17. Then, an intrinsic semiconductor material such as an intrinsic amorphous silicon and, a doped semiconductor material such as a doped amorphous silicon are sequentially deposited on the entire surface of the substrate. These two materials are patterned to form a semiconductor layer 33 and a doped semiconductor layer 35 at the portion at which the gate electrode 11 is formed (FIG. 2b).
A metal including chromium or chromium alloy is deposited and patterned to form a source electrode 21 which contacts one side of the semiconductor layer 33 with the doped semiconductor layer 35 between them. A drain electrode 31 contacts the other side of the semiconductor layer 33 with the doped semiconductor layer 35 between them. The source electrode 21 and drain electrode 31 form ohmic contacts with the doped semiconductor layer 35. Here, when the doped semiconductor layer 35 is between the source electrode 21 and the drain electrode 31 then, the source and drain electrodes are always electrically connected so the TFT cannot act as a switch. Therefore, the doped semiconductor layer 35 between the source 21 and the drain electrode 31 must be removed by continued etching after forming the source and drain electrode. The source electrodes 21 in a row direction are connected to the source bus line 23. The source pad 25 is placed at the end portion of the source bus line 23 (FIG. 2c).
An inorganic insulating material such as silicon oxide or silicon nitride is deposited on the whole surface of the substrate 1 to form a passivation layer 37. The passivation layer 37 is patterned to form a gate contact hole 53, a source contact hole 63, and a drain contact hole 73 which expose some portions of the gate pad 15, the source pad 25, and the drain electrode 31, respectively (FIG. 2d).
An Indium Tin Oxide(ITO) is deposited on the whole surface of the substrate 1 having the passivation layer 37. A pixel electrode 41, a gate pad connector 57, and a source pad connector 67 are formed by patterning the ITO. The pixel electrode 41 is connected to the drain electrode 31 through the drain contact hole 73. The gate pad connector 57 is connected to the gate pad 15 through the gate contact hole 53. The source pad connector 67 is connected to the source pad 25 through the source contact hole 63 (FIG. 2e).
According to the conventional method for manufacturing the LCD panel, the surface of the drain electrode 31 contacts the pixel electrode 41 through the drain contact hole 73 as shown in FIG. 3a. If the etching of the passivation layer 37 for forming the drain contact hole 73 is not sufficient, then a contaminating material 81 such as a residual material of the passivation layer 37 remains on the surface of the drain electrode 31. In this case, the electrical contact between the pixel electrode 41 and the drain electrode 31 becomes unstable. Moreover, because the source contact hole 63 is formed by the same method in the same etching step for forming the drain contact hole 73, the contaminating material 81 may also remain on the surface of the exposed source pad 25, as shown in FIG. 3b. Therefore, the electrical contact between the source pad 25 and the source pad connector 67 may not be stable as well. In particular, the contact resistance becomes higher than normal resulting from the contaminating material 81, so that the screen quality is defective from the distorted voltage signal of the image data.